Benign prostatic hyperplasia (BPH) in man and the dog is testis dependent. Evidence is available in the dog that supports the concept that testosterone and dihydrotestosterone are not the critical factors in development of canine BPH. Indirect evidence in man is compatible with this concept. Evidence suggesting the production of other steroid and protein hormones by the testis is accumulating. Preliminary observations in animals following radiation of the testis indicates no histologic alteration in the Leydig cells and that testosterone production is maintained. Spermatogenesis is severely or completely impaired with some evidence suggesting consequent failure of inhibition of follicle stimulating hormone. We propose to use the dog BPH model to determine whether radiation therapy of the testis, which may inhibit testicular secretion of new androgenic hormones but probably does not alter secretion of testosterone, will prevent or cause regression of BPH. The testes of 5-7 year old Beagles with established BPH will be subjected to 2000 rads. The gross and histologic status of the prostate and testes will be determined prior to the radiation and at four month intervals following it for a year. A second group of Beagles, 2 years old will be subjected to radiation of the testes prior to the onset of BPH and be followed for 2 years. The serum testosterone, dihydrotestosterone androstenediol, estrone, estradiol, estriol, follicle stimulating hormone, leuteinizing hormone, and prolactin levels will be followed periodically during the observation period.